The usefulness of the phenomenon of magnetostrictivity in linear distance or position measuring devices is recognized by the prior art; for example, see Redding, U.S. Pat, No. 4,305,283; McCrea et al, U.S. Pat. No. 4,158,964; Krisst, U.S. Pat. No. 4,071,818; Edwards, U.S. Pat. No. 4,028,619; and Tellerman, U.S. Pat. No. 3,898,555. Common to several of these devices are a magnetostrictive wire which runs in a straight line path through the measurement field, means for inducing a torsional strain at a given position along the wire, and a magnet which is displaceable along the wire, either by connection to a movable body such as a machine tool or by reason of association with a float device. Generally speaking, the position of the magnet represents the position of the monitored quantity and is determined as a function of the time required for a torsional disturbance to propagate from one end of the wire through the area of influence of the magnet or from the position of the magnet to a sensing apparatus located at one end of the wire.
There are problems with devices of this type. This technique depends upon accurate and repeatable measurement of time for determination of the unknown position. Time intervals are often measured via a stable clock and counter or a pulse width to voltage converter. The accuracy and resolution of such time measurements thus depends upon the stability and speed of these measurements circuits. In the case of clock and counter circuits the frequency of the clock generator is generally the limiting factor. Increased resolution of measurement generally requires more expensive circuits which operate at higher speeds and resolution. Thus there is a need in the art for a magnetostrictive linear position determination apparatus which has increased resolution without the increased expense of faster time measurement circuit.